1. Technical Field
This invention pertains to reverse traffic channel circuitry for Code Division Multiple Access (CDMA) based mobile stations and, more particularly, to a method for performing carrier phase offsets at baseband which permits n+1 channels to be added at baseband and modulated with a single quadrature modulator.
2. Background Art
In Code Division Multiple Access (CDMA) based mobile stations, multiple code channels can be used in the reverse traffic channel to increase the data transmission rate. For example, in IS-95B, one fundamental code channel can be used in combination with up to seven supplemental code channels. Under rate set 1, where each code channel has a maximum data rate of 9.6 kbps, the combined data rate can be increased to 76.8 kbps. Under rate set 2, the maximum data rate would be 115.2 kbps The supplemental channels are typically transmitted with different phase offsets of the carrier used to transmit the fundamental channel. The supplemental code channel carrier phase offsets measured with respect to the fundamental carrier for IS-95B are given in Table 1.
TABLE 1 Supplemental code carrier phase offsets Supplemental Code Channel is: Carrier Phase Offset .phi..sub..dagger-dbl. (radiant): 1 .pi./2 2 .pi./14 3 3.pi./4 4 0 5 .pi./2 6 .pi./4 7 3.pi./4
FIG. 1 shows how multiple code channels A.sub.0, A.sub.1, . . . A.sub.n, can be combined with different carrier phase offsets, .phi..sub.i, to produce the RF signal, S(t). Multiple data streams, A.sub.0, A.sub.1, . . . A.sub.n, are separately quadrature spread by pairs of quadrature spreaders 12 (actually modulo 2 adders) and baseband filtered by pairs of filters 14 before being quadrature modulated with the appropriately offset carrier by pairs of quadrature modulators 18. Note that the Q-channel output from the spreader 12 has a 1/2 PN chip delay 22 (=406.9 ns). If the reverse traffic channel were implemented as suggested in FIG. 1, a D/A converter 16 would be required for the output of each baseband filter 14 along with n+1 quadrature modulators 18. Only if there are no carrier phase offsets between the supplemental channels and the fundamental channel can the outputs of the n+1 in-phase baseband filters be summed together as can the outputs of the n+1 quadrature baseband filters. In such case, a single quadrature modulator can be used to produce S(t).
From a cost standpoint it would be desirable if only a single D/A converter and quadrature modulator were required regardless of the carrier phase offsets.